Hydraulic drive systems are utilized in construction equipment such as hydraulic excavators, backhoe loaders, and end loaders. Known systems typically use a plurality of open center control valves to controllably actuate the various hydraulic actuators on the vehicle. Normally, such drive systems are controlled through a series of operator control levers which are mechanically or hydraulically coupled to the control valves. The open center control valves give the system a variable response which is dependent on the load on the actuator. In manually operated systems, this may be desirable because the variable response gives the operator an indication of the load on the actuator. The operator then has a better feel for the operation of the vehicle and can better adjust his/her manipulation of the control levers to achieve the desired result.
However, hydraulic systems employing open center valves are inherently inefficient. The reason for this inefficiency is that there is always some flow from the pump to the tank through each valve. For example, during periods where there is no load upon the hydraulic system, such systems are typically destroked to some minimal level (e.g., 16%). However, some amount of flow must be maintained.
Furthermore, recently a lot of effort has gone into automating or semi-automating the functions of such vehicles. In these automatic or semi-automatic systems, the control characteristics of an open center valve is almost always undesirable. Such systems require consistent control characteristics to ensure constant and predictable operation. One way to achieve constant and predictable results is to use a pressure compensated closed center valve. Pressure compensated valves use pressure feedback to achieve consistent control characteristics. However, the operator loses the sense or "feel" of the load. Furthermore, the operator loses the ability to effectively modulate the pressure, and therefore the operator's efficiency decreases.
However, in such systems it has been found desirable to have drive systems which can exhibit both control characteristics. For example, for a system adapted to perform in manual and automatic modes, it may be desirable to have certain hydraulic circuits operating with open center control characteristics in the manual mode and operating with pressure compensated closed center control characteristics in the automatic mode.
The subject invention is directed at overcoming one or more of the problems as set forth above.